1. Field of the Invention
The present invention relates to a catalytic conversion process known as hydrotreating involving hydrogen and hydrocarbons containing heteroatoms such as nitrogen and sulphur. More particularly, the invention relates to an improved process for removing sulphur and nitrogen compounds from middle distillate hydrocarbon streams wherein the chemical hydrogen consumption is significantly reduced. Still more particularly, the invention relates to a process where the hydrotreated hydrocarbon stream is heated and passed through a high-temperature post-reactor thereby lowering the net amount of hydrogen consumed in the hydrotreating process.
2. Description of Prior Art
Hydrotreating of hydrocarbon streams is carried out to reduce the amount of sulphur and nitrogen compounds in a hydrocarbon stream. Hereby, the impact of these compounds that upon combustion in an engine may form SOx and NOx is significantly reduced. As many countries are tightening the specifications for allowed sulphur content in transportation fuels, it is increasingly important to cost-effectively perform desulphurization of petroleum fractions.
A very significant operating cost related to hydrotreating is the production of hydrogen gas. Typically, hydrogen is produced by steam reforming of natural gas or as a by-product from gasoline reforming (platforming). In the hydrotreating process, hydrogen is consumed not only by hydrodesulphurization, hydrodenitrogenation and hydrodeoxygenation reactions but also by saturation of aromatic hydrocarbons. Furthermore, hydrocracking reactions and saturation of olefinic hydrocarbons are also taking place in the hydrotreating reactor consume hydrogen. Typically, saturation of aromatics causes the greatest contribution to the total hydrogen consumption.
In order to comply with more stringent specifications for the content of sulphur in the refinery products, it is necessary to increase the conversion of sulphur in the feed. Typically, this will mean that the amount of hydrogen used in the process will increase. As the supply of hydrogen is expensive and may be scarce, it is desirable to increase the conversion of sulphur with little or no extra hydrogen consumption.
Typical conditions for a hydrotreating reactor are hydrogen pressures of 15-100 bars, liquid hourly space velocity of 0.5-4 m3 oil/m3 catalyst/h, and temperatures ranging from 310° C. to 400° C. The exact conditions will depend on feedstock type, the required degree of sulphur removal and the desired run length. Typically, the reactor temperature is initially (start of run) at the lower end of the above range, and as the catalyst deactivates the reactor temperature is raised to compensate for loss of activity. When the design temperature for the reactor is approached, the run is normally ended. The design temperature is decided by the metallurgy of the reactor.
The chemical consumption of hydrogen in the hydrotreating process is not constant from start of run to end of run. Typically, at the lower start-of-run temperature, the hydrogen consumption is higher since the saturation of aromatics is an exothermic process that is favoured at low temperatures. Thus, chemical equilibrium between aromatics and hydrogenated counterparts dictates a relatively high degree of saturation and therefore relatively high hydrogen consumption. As the run progresses and the reactor average temperature is raised, the reaction slowly shifts towards lesser aromatics saturation, and therefore the hydrogen consumption typically decreases during the run with the product sulphur content being more or less constant.